1. Field of the Invention
The invention relates to a shift control device and a shift control method of an automatic transmission that performs a downshift through the engagement switch between friction engagement devices. In particular, the invention relates to a technology that shortens the required time in making a determination of the end of engagement of an engagement-side friction engagement device on the basis of the input shaft rotation speed.
2. Description of the Related Art
An automatic transmission that establishes a plurality of gear steps of different speed change ratios by selectively engaging a plurality of friction engagement devices is widely used in motor vehicles and the like. An example thereof is a technology described in Japanese Patent Application Laid-Open Publication No. HEI 9-60717 in which when a downshift is performed by the engagement switch between a release-side friction engagement device and an engagement-side friction engagement device, the fashion of change in the oil pressure on the friction engagement devices differs depending on whether the downshift is a power-on downshift or a power-off downshift. Specifically, at the time of a power-on state, the input shaft rotation speed rises on its own whereas at the time of a power-off state, the input shaft rotation speed does not rise on its own. Therefore, generally, the power-on downshift is performed while the sharp rise in the input shaft rotation speed is restrained by the release-side oil pressure. On the other had, the power-off downshift is generally performed by releasing the release-side friction engagement device early and raising the input shaft rotation speed by the engagement-side oil pressure. Besides, as shown in Japanese Patent Application Publication JP-A-2001-65680, there occurs a case where an accelerator-on operation is performed halfway through a power-off downshift. In such a case, the downshift is performed while the sharp rise in the input shaft rotation speed is restrained by a hydraulic control of the engagement-side friction engagement device or a torque control of the motor.
Incidentally, when it is determined that the input shaft rotation speed is in a neighborhood of the synchronous rotation speed of the post-shift gear step, an engagement end determination indicating that the engagement of the engagement-side friction engagement device has ended is made, and then the shift control is ended by, for example, raising the oil pressure to a maximum value, or the like. In that case, in order to prevent a false determination from being made regarding the engagement end determination, the engagement end determination is made provided that the determination continues to hold over a predetermined time. For example, in the case of the power-on downshift, since the shift is performed while the sharp rise in the input shaft rotation speed is restrained by the release-side oil pressure, it is not immediately distinct whether the dwell of the input shaft rotation speed in the vicinity of the synchronous rotation speed has been caused by the release-side oil pressure or the engagement-side oil pressure. Therefore, there is a need to confirm that the input shaft rotation speed is kept in the vicinity of the synchronous rotation speed despite a further drop of the release-side oil pressure, before making the engagement end determination. Thus, there is a need to set a relatively long determination time.
However, if the determination time is set long in order prevent a false determination, the shift time becomes long. Therefore, in a case where an accelerator-on operation is performed during a power-off downshift, the shift requires a time despite the driver's request for a driving power, that is, it takes a long time until a desired driving power is obtained, causing an uncomfortable feeling to the driver. In particular, in a case where such an accelerator-on operation has caused a judgment for a multiplex downshift that involves a further downshift to a gear step that is lower than the gear step that needs the engagement switch between friction engagement devices in order to input torque from the motor, a downshift to an intermediate relay gear step is first performed, and after the shift control to the relay gear step ends, the next shift is performed. Therefore, the time of dwell at the driving power of the relay gear step becomes long if the aforementioned determination time is long. Thus, the aforementioned problem becomes conspicuous.